The 16-year-old victim from Bulkeley High School in Hartford is expected to recover.
Police say he was stabbed in the stomach during a fight with a student from another school.
Deputy Police Chief Brian Foley says the two got into a fight in the back of the school, and the victim was stabbed in the abdomen.
Two people were taken into custody and questioned, but both were later released and no one has yet been charged.
Foley said the injured teen was conscious and in stable condition at Connecticut Children's Medical Center.
It's not clear what prompted the fight.